1. Field of the Invention
This invention generally relates to a split beam laser welding apparatus and, more particularly, to a split beam laser welding apparatus for welding an improved spacer grid assembly.
2. Description of the Prior Art
Nuclear fuel assemblies are customarily of the rod type where the fuel rods are held in a separate array by a spacer grid assembly. FIG. 1 illustrates a prior art spacer grid assembly 8, such as the one disclosed in U.S. Pat. No. 4,879,090. The spacer grid assembly 8 is comprised of orthogonal sets of interior strips 10 having mixing vanes 11 as appendages. The interior strips 10 have supports 12 for the fuel rods and are joined together by a plurality of upper intersection welds 13 and lower intersection welds 14. Prior to welding, sacrificial tabs 15 are constructed at the intersection of the interior strips 10.
As discussed in U.S. Pat. No. 5,179,261, which is hereby incorporated by reference, to perform the upper intersection welds 13, cutouts for laser beam access were made at the base of the mixing vanes 11 rather than along their sloping surfaces as in other designs. These cutouts thereby minimized the detrimental effect of degrading the Departure from Nucleate Boiling (DNB) performance that is created by flow turbulence produced at the intersection welds.
A split beam laser welding system was disclosed in U.S. Pat. No. 5,179,261 to overcome the access problems associated with welding this design. This welding system, as shown in FIG. 2, may be comprised of a bifurcating mirror 25 for dividing the beam into a plurality of beam segments and plane mirrors 26 for routing the beam segments 27. Focusing lenses 22 are used to then direct the laser beams into different quadrants of an intersection of the strips 10. The beams may be moved down the strips 10 to induce a flow of molten metal. The resultant weld with this split beam laser welding System can thus be made without melting the mixing vanes 11.
An improved spacer grid assembly is disclosed in co-pending U.S. application Ser. No. 08/027,756, which has a common assignee to the present application. This spacer grid assembly, as shown in FIG. 3, comprises strips that are formed in an "egg crate" arrangement with strips intersecting each other at right angles. A slotted cylindrical member is placed at each intersection so that the cylindrical member encircles the intersection of strips. The tops of the cylindrical members can be shaped or formed with vanes that induce swirl flow within flow channels defined between the fuel rods or deflect part of the flow in one channel into an adjacent channel. This grid may significantly improve the mechanical strength, hydraulic resistance, and thermal performance of the assembly. Other advantages of this improved spacer grid assembly are disclosed in the above-referenced application.
With the improved spacer grid assembly, however, a large number of welds need to be performed in order to fabricate the spacer grid assembly. While the previous spacer grid assembly only required an upper side weld 13 and a lower side weld 14 at each strip intersection, the improved spacer grid assembly may require up to five different welds on each side of the assembly to ensure structural integrity and to achieve maximum mechanical performance from the spacer grid assembly.
For instance, with reference to FIG. 4, a single weld may be required at the intersection of two flat strips. In addition, four welds at each intersection may be necessary to affix the slotted cylindrical tube to the flat strips. Since an additional five welds would be required at the other side of the spacer grid assembly, the improved spacer grid assembly may require up to ten welds for each intersection of the flat strips, as opposed to two welds with the spacer grid assembly shown in FIG. 1.
Using standard welding techniques, this large number of welds would increase the fabrication time significantly since the welding source or the grid assembly itself would be repositioned numerous times for each intersection. Moreover, since the cylindrical members extend above the strips, the welding system or the spacer grid assembly would have to be tilted or rotated to several different orientations in order to perform the welds which join the cylindrical member to the strips. Thus, it would be a problem in the prior art to fabricate the improved spacer grid assembly using standard welding techniques.